The present invention relates to coatings applied to metal components exposed to high temperature environments, such as gas turbine engines, radial inflow compressors and radial turbines, including micro-turbines and turbo-chargers. In particular, the present invention involves a new type of abradable coating containing very small ceramic microspheres in the form of a microballoon dispersion metal matrix composite (“MDMMC”) which is applied to the surface of metal components. The new abradable coating allows for very close clearance control between the bucket tips and shroud in gas turbine engines, thereby reducing hot gas leakage and improving overall gas turbine efficiency. High temperature abradable coatings embodying some of the same general concepts as the present invention are described in commonly-owned application Ser. No. 09/863,760, the disclosure of which is hereby incorporated by reference.
The present invention also includes a method for applying the new abradable coatings to turbine shrouds in order to improve the long-term performance and efficiency of the turbine blades without requiring that the blades be tipped. Although the invention has been found particularly useful in stage 1 gas turbine engine shrouds, the same abradable coating compositions can be used in any stage of gas turbine engines, as well as on hot gas path metal components of other rotating equipment exposed to high temperature environments. The invention can also be used to repair and/or replace the coatings on metal components already in service, such as turbine shrouds.
It is well known that the high temperatures encountered in early stages of gas turbines creates various problems relating to the integrity, reliability and life expectancy of components coming in contact with the hot gas, particularly the rotating buckets and turbine shroud. One objective of the present invention, like the abradable coatings described in application Ser. No. 09/863,760, is to enable the shroud coating to cope with the high gas temperatures for much longer periods of time while maintaining tight clearances at bucket tips. In order to achieve maximum engine efficiency, the buckets must rotate freely within the turbine housing (shroud) without interference and with the highest possible efficiency relative to the amount of energy available from the expanding working fluid. The highest efficiencies are achieved by maintaining a minimum threshold clearance between the shroud and the bucket tips to thereby prevent unwanted “leakage” of the working fluid over the tips of the buckets. Increased clearances due to premature or excessive bucket wear ultimately result in significant decreases in overall efficiency of the gas turbine engine. Thus, only a minimum amount of leakage of the hot gases at the outer periphery of the buckets, i.e., the small annular space between the bucket tips and turbine housing, can be tolerated without sacrificing engine efficiency.
The need to maintain adequate clearance without significant loss of efficiency is made more difficult by the fact that as the turbine rotates, centrifugal forces acting on the turbine components as well as high operating temperatures cause the buckets to expand radially in the direction of the shroud. Thus, it is important to establish the lowest effective running clearances between the shroud and bucket tips at the maximum anticipated operating temperatures of the working fluid.
In the past, various types of abradable coatings have been applied to the turbine shroud to help create a minimum running clearance between the shroud and bucket tips under steady-state temperature conditions. Typically, such coatings have been applied to the surface of the shroud opposite the buckets using a material that can be readily abraded by the tips of the buckets as they turn inside the housing at high speed with little or no damage to the bucket tips.
A number of design factors must be considered in selecting an appropriate material for use as an abradable coating for a shroud, depending upon the environmental coating composition and properties, substrate material composition/properties, the specific end use, and the operating conditions of the turbine, particularly the highest anticipated working fluid temperature. Ideally, the cutting mechanism (e.g., the bucket blade tips) is sufficiently strong and the coating on the shroud sufficiently brittle at high temperatures to abrade without causing damage to the bucket tips themselves. That is, at the maximum anticipated operating temperatures, the shroud coating should preferentially abrade in lieu of any loss of metal on the bucket tips.
Commonly-owned G.E. application Ser. No. 09/863,760 discusses another important design factor to be considered in the context of abradable shroud coatings, namely the rate of degradation, e.g., oxidation, of the coating due to exposure to hot gases containing oxygen over long periods of time at elevated temperatures. Most prior art coatings, e.g., ceramic abradable coatings, are quite dense and thus require additional bucket tip reinforcement to make them abradable. Another problem relates to the relationship between coating abradability and resistance to oxidation in higher temperature applications. As the gas temperature increases, coating structures become more and more ductile. This increased ductility tends to reduce the ability of the coating to be abraded. Most prior art abradable coatings use higher levels of porosity to compensate for this increased ductility and yet maintain abradability at high temperature. However, the higher porosity tends to reduce the life span of the coatings at high temperatures because the same porosity volume that make the coatings abradable also renders them much more vulnerable to oxidation, particularly in the earlier turbine stages.
Various prior art patents describe abradable coatings for use in turbocompressors and gas turbines. application Ser. No. 09/863,760 describes a coating system having two components: (1) a “fugitive” polymer or other plastic phase (such as polyester or polyimide) that can be burned off without leaving any residue or ash to create a porous coating; and (2) a brittle intermetallic phase, such as β-NiAl or an intermetallic phase former that has superior oxidation resistance as compared to MCrAlY, where M can be CoNi, Fe or Ni. This second component serves to increase the brittle nature of the metal matrix, thereby increasing the abradability of the coating at elevated temperatures. An alternative third phase can also be used, namely, a metallic oxidation-resistant matrix phase such as MCrAlY, e.g., Praxair Co211 (Co32Ni21Cr8Al0.5Y), NiCoCrAlY, FeCrAlY or NiCrAlY, e.g., Praxair Ni211 (Ni22Cr10Al1Y).
A number of other abradable coatings have been used in the past on compressor shrouds and gas turbine components. See, e.g., U.S. Pat. Nos. 3,346,175; 3,574,455; 3,843,278; 4,460,185, 4,666,371 and 5,472,315. Unfortunately, these conventional coatings are not sufficiently durable or resistant to oxidation in higher temperature environments. The prior art coatings tend to oxidize, delaminate or even separate from the shroud substrate as the turbine undergoes thermal cycling during startup and shut down. The poor oxidation resistance of many prior art compositions may be attributable in part to the relatively high porosity levels (about 55% by volume) in the abradable top coat which tend to allow a much higher rate of ingress of oxygen into the coating.
One improved prior art coating known as Sulzer Metco SM2043 consists of MCrAlY together with 15 wt % polyester and 4 wt % boron nitride (hBN). (See U.S. Pat. No. 5,434,210). The MCrAlY component of the SM2043 nominally contains CO25Nil6Cr6.5Al0.5Y and is recommended for applications at approximately 1380° F. without tipped (uncoated) buckets, and up to 1560° F. for tipped buckets. Because the SM2043 material does not abrade well above 1380° F., it can result in non-uniform wear of the shroud coating and/or cause damage to the bucket tips themselves by the rotational impact of the bucket with the shroud metal, ultimately requiring some type of tip reinforcement or coating. In addition, because of the high porosity in coatings using Sulzer Metco SM2043, the oxidation life of the coatings is relatively short at operating temperatures above 1560° F. For example, they begin to show lower oxidation resistance at temperatures above 1380° F., and the resistance level deteriorates significantly above that temperature, with many coatings lasting only a few hundreds or thousands of hours at temperatures approaching the level of early turbine stages (1700° F.), this is one or two orders of magnitude less than the required hours.
Thus, for many years, a significant need has existed in the art for an improved abradable coating for gas turbine shrouds operating at higher than average temperatures, i.e., above 1380° F., which is capable of achieving a longer oxidation life, preferably up to or beyond 24,000 hours, when used at gas temperatures in the 1600-1800° F. range. There is also a significant need for abradable coatings capable of ensuring that the turbine buckets suffer from only minimal wear during startup and shutdown due to cyclic radial expansion and contraction of the turbine components. A need also exists to provide a strong, but abradable coating that will avoid the necessity for tipped blades which might otherwise be required due to the generally non-abradable nature of coatings in the higher temperature ranges of turbine shrouds. Finally, a need exists to provide a coating that will have sufficient erosion resistance over the full anticipated life of the gas turbine equipment, thereby avoiding the need to interrupt operation to maintain and/or prematurely replace the coating.